1. Field of the Invention
The present invention relates to a solid state imaging device and an imaging apparatus.
2. Description of the Related Art
Heretofore, a vertical overflow drain solid state imaging device has been proposed in which excess electrons in a light receiving part are drained to the substrate side and excess holes are evacuated to a ground part (GND) from a channel stop part provided around individual photosensors on the substrate surface through a P-type region disposed at a deep place in the substrate.
Moreover, a structure has been disclosed in which excess holes are partially drained to GND out of a channel stop part around individual photosensors on the substrate surface (hereinafter, referred to as a route A), the remaining excess holes are moved in an overflow barrier area, the excess holes are transferred to the outer part of a solid state imaging device through the overflow barrier area and the holes are evacuated to the GND connected to the outer part (hereinafter, referred to as a route B) (for example, see Patent Reference 1 (JP-A-2001-15729)). In the case of this structure, since a voltage drop occurs in a place apart from the ground part, holes are delayed to migrate in the photosensor, and the overflow barrier potential on route B fluctuates in the surrounding area and the center part of an effective imaging area to cause shading in saturated signal electrons, which leads to a degraded image quality of a taken image. In other words, in order to solve this problem, it is necessary to reduce the resistance in the overflow barrier area on the route B that is the path of the excess holes to suppress the voltage drop caused by holes passing through the route B and to suppress the overflow barrier potential from fluctuating in the center part and the surrounding area of the image area.
In order to reduce the resistance in the overflow barrier area, increasing the impurity concentration in the overflow barrier area has been considered. However, in this case, it is difficult to empty the overflow barrier area because the overflow barrier area is neutralized, which leads to a problem of blooming when a large light quantity enters.
On the other hand, in the case in which the impurity concentration in the overflow barrier area is decreased, the overflow barrier area can be emptied out because the potential in the overflow barrier area rises, and thus the problem of blooming when a large light quantity enters can be improved. However, this causes a problem that the resistance in the overflow barrier area is increased to cause shading when a large light quantity enters.
In other words, for the invention described in Patent Reference 1, since there are trade-offs between the overflow barrier area being emptied out and a reduced resistance, there is a problem that it is difficult to suppress both the shading and blooming in saturated signal electrons.
On the other hand, in the solid state imaging device, with the scale down of the pixel size because of the advance in multipixels and miniaturization, the spacings between pixels in the vertical direction and in the horizontal direction are increasingly narrowed. On this account, in the structure of the channel stop areas formed only on the surface of a semiconductor substrate, it is difficult to efficiently prevent a phenomenon of photoelectrically converted electric charges mix into the adjacent pixels in the photosensor part (hereinafter, referred to as color mixture). Therefore, in order to prevent this color mixture, it is necessary to form a P-type region below the channel stop area between pixels and a vertical charge transfer part (in the vertical charge transfer part, the area deeper from the semiconductor substrate surface than the vertical charge transfer channel formed in an N-type region) to a deeper area in the semiconductor substrate in the depth direction.
Then, a method is disclosed in which a P-type impurity to form a channel stop area is formed by using a plurality of ion injections with different injection energies (for example, see Patent Reference 2 (JP-A-2004-165462)). However, in the case of using this method, the impurity concentration becomes non-uniform in the depth direction, and is difficult to form a channel stop area of uniform impurity concentration to a deeper area. In addition, when the pixel size is scaled down, a so-called narrow channel effect occurs in which a P+-diffusion layer configuring the channel stop area around each of the photosensors narrows the path of electric charges.
In the case of using the method described in Patent Reference 2, as described above, the impurity concentration of the channel stop area becomes non-uniform in the depth direction, and in addition to this, the narrow channel effect occurs causing the potential in the area to drop, and the minimum point of the potential in the overflow barrier area is shifted to a position shallow from the substrate surface.
Therefore, in the case in which the pixel size is small, it is difficult to form the overflow barrier area in a position deep from the substrate surface. In addition, desirably, the overflow barrier area is formed in a position deeper to some extent from the substrate surface in order to widen the area to be photoelectrically converted by the light incident into the photosensor for improved sensitivity. However, as described above, in the case in which the pixel size is small, it is difficult to form the overflow barrier area at a deep position, which leads to a problem that it is difficult to allow a solid state imaging device to have higher sensitivity.
Moreover, when the channel stop area becomes non-uniform in the depth direction, the potential becomes wavy in the depth direction, which is not preferable to drain excess holes out of the channel stop area to the front surface side of the substrate.
The problems of difficulty in higher sensitivity and the a nonuniform channel stop area becomes particularly problematic when the pixel size is 2 μm or below.
In addition, the method described in Patent Reference 2 has a problem that the number of process steps is increased. Moreover, it is necessary to form a thicker ion injection mask formed of a resist because it is necessary to form the channel stop part by high energy ion implantation, which causes a difficulty of microprocessing a thick resist film, which also leads to a problem that it is difficult to form pixels finer.
In addition, Patent Reference 1 discloses the invention in which such a structure is formed that the channel stop part is formed from the silicon substrate surface to a deeper position than the photosensor, whereby holes to be moved along the route B are partially drained from the channel stop part formed at a deeper part to the GND on the front surface side of the substrate (route C).
As described above, in the case in which the pixel size is scaled down, the sensitivity is also dropped even though the method described in Patent Reference 2 is used, and thus the sensitivity is dropped even though the method is adapted to the invention described in Patent Reference 1. Therefore, in this case, it is difficult to attain the prevention of a color mixture as well as a higher sensitivity. Thus, in the case in which the method described in Patent Reference 2 is adapted to the invention described in Patent Reference 1, there is a problem that it is difficult to scale down the pixel size.
On the other hand, for a scheme of preventing the sensitivity from being dropped in the case in which the pixel size is scaled down, electric charges obtained by photoelectric conversion in the area other than the light receiving part, that is, electric charges obtained below the vertical transfer part can be considered for use as signal electric charges. In order to allow electric charges obtained by photoelectric conversion below the vertical transfer part for use as signal electric charges as well, for example, Patent Reference 3 (JP-A-2004-356157) discloses a structure in which a twin P-well structure is formed such that a second P-well region is formed below the first P-well region through an N−-type impurity area. A channel stop part for pixel separation is also formed deeper in the position of the second P-type well region and is electrically connected to the P-type well region. Also, in this case, it is necessary to form the channel stop part deeper. On this account, even though the method described in Patent Reference 3 is adapted, it has a problem similar to the case in which the method described in Patent Reference 2 is adapted to the invention described in Patent Reference 1 as described above.